Hybrid air–water cooling of avionic pin–fin heat sinks with dense microchannels: thermal gains versus hydraulic costs

Efficient thermal management is critical for the stability, reliability, and operational safety of high-power-density avionic electronics, especially under strict weight and space constraints. This study numerically investigates hybrid pin–fin heat sinks combining top-surface air impingement with internal water flow to enhance cooling performance under moderate (20,000 W/m 2 ) and high (40,000 W/m 2 ) heat fluxes. The novelty of the proposed design lies in replacing each conventional 2 mm circular water channel with an array of 25 dense microchannels (0.2 mm diameter each), significantly enhancing convective heat transfer by increasing the heat transfer surface area and promoting more uniform internal flow. Five heat sink configurations were analyzed using three-dimensional CFD simulations at different air and water velocities, varying in internal channel geometries and water-cooling conditions, while pin-fins were present in all cases except one configuration designed to isolate their effect. Results show that the dense microchannel configuration (HS–PF–CH-P) achieves the lowest base (21.7–21.9 °C) and centerline (20.8–20.9 °C) temperatures and the highest local heat transfer coefficients (~ 15,400–23,370 W/m 2 ·K), indicating superior thermal performance. However, this thermal gain comes at the cost of significantly higher pumping power, as quantified by the performance evaluation criterion (PEC = 0.43 at V water = 0.5 m/s), highlighting the inherent thermo-hydraulic trade-off. In contrast, the pin–fin configuration with conventional channels (HS–PF–CH-C) provides a balanced performance, with reduced thermal resistance and PEC > 1, demonstrating a more energy-efficient alternative. Overall, the study establishes that integrating dense microchannels with pin-fins under hybrid air–water cooling substantially enhances thermal performance and hotspot mitigation, while emphasizing the trade-off between heat transfer enhancement and hydraulic cost, offering guidance for the design of compact and reliable avionic heat sinks.